Reflector arc lamp for the projection of colored pictures



OF COLORED PICTURES Feb. 1 1938. GRETENER REFLECTOR ARC LAMP FOR THE PROJECTION Filed Feb. 6, 1.956

Patented Feb. 1, 1938 UNITED STATES REFLECTOR ARC LAMP FOR. THE PROJEC- TION OF COLORED PICTURES Edgar Gretener, Berlin-Sicmensstadt, Germany,

minor to Opticolor Aktiengeaellachaft,

Glarus, Switzerland, a Swiss corporation I Application February 6, 1936, Serial No. 62,592

In Germany February 1, 1935 1 Claim.

My invention relates to an arcla'mp, and more particularly to a reflector arc lamp for the pro- Jectlon of colored pictures that may be advantageously applied also to projectors and similar devices. To attain a true reproduction of colored films, particularly of reticulated films, the light source for the projection, as a rule, calls for a particularly high degree of brightness and uniformity of the distribution of the light emitted. To attain the necessary uniform and stable illumination of the film gate when projecting the crater of the positive electrode onto the film gate the brightness in the crater of the electrode must be as uniform as possible.

This may be accomplished according to the invention by the simultaneous use of a sym-' ticularly large cross-section, which is particularly disadvantageous from an economical point of view, since the consumption of energy is con-- siderably increased thereby. In this case only a small portion of the electrode was luminous, whereas the other portions of the tip of the electrode were projected on the portion surrounding the film gate, so that the latter was heated.

unnecessarily.

According to the invention the above-mentioned drawbacks are removed and a source of light as homogeneous and stable as possible is obtained by the use of magnetical longitudinal fields in guiding the arc and by limiting the luminous surface with the aid of a tightly disposed sleeve. In this manner by the. concentration of the arc discharge a stable arc of great length is attained as well as a uniform consumpface so that also only the latter emits the light. Besides effecting a saving in energy the further advantage is obtained in that the development of heat is prevented. The heat radiating from the luminous surface is, moreover, absorbed to a great extent by the cooled sleeves. Furthermore, the ends of the electrodes burn off in such a manner that the burning surface presents only a very shallow crater and a rounding of! of the electrodes is also eliminated, even if the carbon pencils are particularly overloaded.

The guide ofthe arc is preferably effected by the use of a current-carrying coil. The coil is arranged in such a manner that the axis of the magnetic field coincides substantially with the direction of the axis of the arc discharge.

The coil may, for instance, be arranged behind the mirror of a reflector arc lamp, in the optical axis of which are arranged both electrodes. It is is, however, advantageous to place the coil as close as possible to the discharge. In this manner the same strength of field with a considerably smaller number of ampere turns may be obtained than has hitherto been possible. In this case the coil may be either so arranged as to concentrically surround an electrode or it may be divided into various parts which are distributed overboth electrodes or these parts may be so arranged that the resulting field coincides substantially'with the axis of the discharge.

By such an arrangement it is possible to produce the magnetic field directly at the point at which it is needed. The magnetic field may easily be so distorted by the considerable iron masses of the lamp or casing that it serves its purpose only insufliciently or not at all, insofar as the stabilizing action of the field depends largely upon its adjustment. 0n the other hand, it may be of advantage to diminish the influence of the other iron masses or electric fields or to strengthen the magnetic flux or to develop the magnetic field by a particular shape of the sleeve; for instance, by only a partly closed sleeve in such'a manner that the magnetic field assumes a particularly suitable form.

The coil is so intensely heated by the heat of the arc and by the current flowing therein that it is of advantage to provide also a cooling for the coil, preferably by water. In this case the coil may be directly coo1ed,'f0r instance, by water which flows through the coil or the cooling may be effected indirectly by a good heat contact with cooled metallic masses; for instance, with the sleeve of the anode. The electrode is in direct contact with a sleeve, over which a coil is tightly by a narrow gap.

The coil arranged in the neighborhood of the electrode is particularly suitable to be designed as series coil; the lamp current is caused to flow directly through a relatively small number of turns having a thick cross-section and produces in this manner the magnetic field which is employed for the stabilization without the necessity of any special auxiliary means. Particular leads are also dispensed with which cause an unnecessary loss of light, since they lie in the path of the rays. The coil may, of course, also be traversed only by a portion of the lamp current either to supply still another coil with the residual current or to influence thereby the magnitude and the direction of the magnetic field. It may also be of advantage to utilize the voltage, by which the lamp is operated for maintaining the coil current (shunt) or for combining this method of operation with the series system in order to attain the desired relationship between the field and the operating current and are voltage.

It has further been found that the symmetrical magnetic field is particularly advantageous if it possesses radially directed intense components, especially if the arc lamp is used for the projection of moving pictures. While'the magnetic field in the axis of the discharge runs in parallel relation to the discharge the strength of the magnetic field outside of this axis has a direction which is inclined with respect to the discharge. The action of this particular formation of the field will be understood by the following test:

On examining the luminous surface of the positive electrode stroboscopically, bright and dark portions revolving on the luminous surface will be perceived. Since in the case of the projection of moving pictures a stroboscopic effect also occurs by the use of the rotating diaphragm the above-mentioned inhomogeneity may interiere with the illumination to a considerable extent. The period of rotation of this inhomogeneity'with respect to the frequency of pictures per sec. must be fixed, so that the bright surface appears homogeneous. This may be accomplished according to the invention by intense radial components of the magnetic field. The inhomogeneity should revolve, for instance, in the case of the normal frequency of 24 pictures per sec, at least once, preferably several times within 1/48 sec. When inserting dark intervals for reducing the flickering to a minimum, the speed of rotation must be changed accordingly. Such a homogenization is not only of importance for the projection of moving pictures but it may, as a rule, be employed for a variety of purposes.

The uniformity of the luminous surface depends upon the intensity of the radial field components, besides upon the arc length and current intensity and when forming this field care should be, therefore, taken to have a sufllciently intense radial component along every point of the discharge path, while maintaining the symmetrical distribution within strict limits. The magnetic field may, for instance, be produced by a coil concentrically disposed with respect to an electrode, the coil being preferably arranged between two cylinders of magnetic metal. In this manner a longitudinal field is formed in the axis of the discharge, whereas the lines of force formed away from the axis of the discharge are bent towards the outer jacket and cause the formation of the radial components. This radial aromas field is particularly strong, if the coil is magneticaily closed at its back side so as to form a double-walled pot surrounding the electrode. A particularly suitable magnetic field is she created if both the positive and the negative electrodes are surrounded by-a current-carrying coil, the coils being so chosen that the same sign are opposite to one aning drawing.

Fig. 1 is a diagrammatic section through an arc lamp, showing the arrangement of a stabilizing coil.

Fig. 2 is a diagram showing the use of magnetic 1materials for the production of the magnetic eld.

Fig. 3 is a view similar to Fig. 1, illustrating the assembly of the sleeve and coil to a unit.

Fig. 4 is a view similar to Fig. 1, showing the coil arrangement in which the electrodes are inclined with respect to each other.

Fig. 5 is a view similar to Fig. 1, illustrating a particslar suitable type of an arc lamp according to the invention.

Fig. 6 is a section through one electrode, showing a suitable form for reflector arc lamps.

In Fig. 1, i and 2 denote the two carbon electrodes between which the arc is formed. Both electrodes lie in the optical axis of the mirror H which reflects the light of the arc. The stabilizing coil 8 is arranged concentrically with respect to the electrodes behind the mirror so that the magnetic field is also symmetrical.

Fig. 2 shows an embodiment in which both carbon electrodes i and 2 are surrounded by a sleeve 3 of magnetic material. The field is produced by the coil 8 and extends between the electrodes in the direction of the arc.

Fig. 3 shows also an arc lamp, the positive electrode of which is surrounded by a coil I, in the core 4 of which water circulates. The winding 8 is arranged directly on the water-cooled sleeve 3. The negative electrode 2 is not cooled, but it has an iron jacket I! of particular form which concentrates the magnetic field on the tips of the electrode 2 and causes the formation of the radial components. 8 are the leads for the coil current.

Fig. 4 shows a further embodiment in which the positive electrode i is surrounded by a coil l consisting of tubes in which water is circulated. Through these tubes the lamp current is supplied. The support 8 of the coil is designed as a sleeve tightly disposed around the electrode i, and protects the electrode fromconsumption and centers the same. The portion in direct contact with the carbon consistsof iron in order to crowd the field together and extends so far along the electrode I as to leave a narrow gap II at the front end between the sleeve 3 and the electrode i, the gap causing a radial stray field and at the same time preventing too intense a cooling of the luminous surface. In order to prevent the discharge from taking place on the sleeve, the front part thereof is insulated by an intermediate layer The negative electrode 2 is also provided with a coil 8 surrounded by an indirectly cooled iron jacket 1 which is open at the front end thereof. In such an arrangement, in which the electrodes are inclined with respect to each other it is essential that the field resulting from the two coils be exactly adapted to the path of current in the arc and that all outer interferences be prevented.

An arc lamp, in which the invention is embodied in a particularly suitable-form is shown in Fig. 5. The positive electrode I is surrounded by a sleeve 3 preferably made of steel. The sleeve is cooled by water and protects the jacket surface of the electrode. The sleeve is provided with the coil 8 which is surrounded by a pot 1 consisting of iron. Owing to the action of the coil and the particular shape of the pot a magnetic stray field of high intensity is formed which is symmetrical with respect to the axis of the electrode 1 and which is radially directed to a great extent particularly in the immediate neighborhood oi the luminous end face of the positive electrode. Further in Fig. 5 a device is shown which is of particular value in conjunction with cored carbons and which permits of a stabilization of the arc beyond the magnetic action. That is to say, a strong converging current of air is blown into the are from the air chamber 5 through the nozzle 6 and strikes the negative electrode 2. The current of air passes then through the casing l3 and the chamber l6 and effects a complete combustion of the rare earths present in the cored carbons as well as an entrainment of the residues of combustion. The air on entering the branch [3 produces a depression which causes the air to be drawn in from the surrounding air space through the openings IE or through the front opening of the branch. This current of air entrains all the particles ejected from the electrode thus preventing them from depositing on the hollow mirror ll.

However, it is also possible as shown in Fig. 6 to draw in air by a particular pump l8 through the openings I 5 and the suction conduit il in order to create a strong current of air, and thus to prevent with certainty the glowing particles from reaching the mirror.

The supply of the current to the positive electrode is provided by the cable 9, the springs l0 and the brushes H which are pressed against the metal surface of the electrode. It is essential that the current be supplied as close as possible to the tip of the electrode in order to reduce the heat developed in the carbon to a minimum, since otherwise an evaporation of the core material and, under some circumstances, an ejection of material from the jacket may occur.

The cooling of the sleeve is effected by water which flows through the chamber 4. Also the negative electrode 2 may be surrounded by a water-cooled sleeve l2 so as to prevent a premature buming of the electrode.

I claim as my invention:

In an arc lamp comprising positive and negative electrodes, a magnetic coil surrounding the positive one of said electrodes and extending closely adjacent the point thereof, a sleeve closely surrounding said positive electrode adjacent said point, means rapidly cooling said sleeve, and a metallic pot substantially enclosing said coil and having one wall disposed between the coil and the electrode and another wall on the side of the coil away from the electrode.

EDGAR GRETENER. 

